now after the user enters the grade at line 37 you need to validate it. If not valid then make him enter the grade again. You probably need to do all that in a do loop or a while loop, either one will work ok. Something like this sudo code.

while grade is not ok
get grade from keyboard
is grade between 0 and 100
if it is then exit the loop
otherwise if not display an error message
end of while loop